Conventionally, as a load drive device wherein an inductive load such as for example the coils of a solenoid plunger or stepping motor were driven, a device has been employed whereby the mean voltage or mean current applied to the load is controlled by opening and closing a circuit opening/closure means inserted between this load and the power source and consisting of a semiconductor switch or the like. This device is known as a chopper control or PWM (pulse width modulation) control, and a typical conventional circuit layout is shown in FIG. 11 and FIG. 12.
The circuit shown in FIG. 11 is of a circuit construction called a high side switch for switching current on the power source side of the load; the circuit shown in FIG. 12 has a circuit construction called a low side switch that switches current on the earthed side of the load.
FIG. 11 is constituted by providing two transistors TR1, TR2 constituting switch means and two resistors R0, R1; a PWM signal of predetermined duty ratio is applied to the base of transistor TR1, a power source is connected to the emitter of transistor TR2; resistor R0 is connected between the emitter and base of transistor TR2; resistor R1 is connected between the collector of transistor TR1 and the base of transistor TR2; and the emitter of transistor TR1 is earthed. Also, a flywheel diode FD is connected in parallel with the inductive load L that is the subject of drive by this circuit; the cathode of flywheel diode FD is connected to the collector of transistor TR2; and the anode of flywheel diode FD is earthed.
With such a construction, when the PWM signal becomes high level, turning transistor TR1 ON, in response to this, transistor TR2 is turned ON, and the power source is applied to load L through transistor TR2, so that load current flows to earth from the power source through transistor TR2 and load L. When this happens, due to the characteristic of the load L, this load current increases with time, finally becoming constant at a saturation point given by the power source voltage and the amount of resistance, not shown, in inductive load L.
However, when chopper control is exercised, with the object of controlling the mean current flowing in the load, the mean current value required in load L is lower than the current value at the saturation point, so, before the load current flowing in load L reaches this saturation point, the PWM signal goes from high level to low level, causing transistor TR1 to go from ON to OFF and transistor TR2 also to go from ON to OFF, cutting off the power source from load L.
In this connection, the ON/OFF timing of transistor TR2, i.e. the timing of high level/low level of the PWM signal is determined by for example ON/OFF ratio control (duty control) of transistor TR2 based solely on the time or by detecting the load current flowing in load L by means of a current detector, not shown, in accordance with a predetermined standard such as constant-current control based on its instantaneous value or mean value.
In either case, when transistor TR2 turns OFF and the power source is cut off from load L, the load current flowing in load L flows back through flywheel diode FD due to the inductive component of load L, and is gradually decreased by the electrical resistance of the circuit and the forward voltage of the flywheel diode FD.
In this condition, when the PWM signal again goes from low level to high level, causing transistor TR1 to turn ON and transistor TR2 to turn ON, the power source is once more connected to load L and the load current flowing in load L progressively increases as described earlier.
Thus, with this high side switch, the ON/OFF timing of transistor TR2 is altered by means of the ON/OFF timing of the PWM signal applied to the base of transistor TR1, and the load current flowing in load L can thereby be controlled.
The advantages of this high side switch include:
(1) Load L is on the earth side of the switch (transistor TR2), so that voltage is not constantly applied to load L, so, even if a short circuit occurs when load L is not being driven, it is still safe; this arrangement is also on the safe side so far as electrical corrosion etc. due to moisture is concerned.
(2) Also, even if there is a short circuit in the wiring of the load whilst the load is being driven, the load L can be cut off by a switch (transistor TR2).
(3) Return wiring from load L can be dispensed with.
On the other hand, a drawback of this high side switch is that, due to the nature of the circuit, P-type elements such as a PNP bipolar transistor, P channel FET, or P channel IGBT etc. must be employed. In general, P type elements are inferior in characteristics to N type elements such as NPN transistors, N channel FETs, or N channel IGBTs in many respects such as their current amplification factor, voltage-withstanding ability, and saturation voltage, and are moreover costly. The efficiency of the switch circuit is therefore poor and it is uneconomic. And if the circuit is constructed using N type elements such as NPN transistors, N channel FETs or the like instead of the P type elements, the circuit construction will normally be an emitter follower or source follower circuit; the efficiency of this is poor in that the base-emitter voltage or gate-source voltage directly affects the collector-emitter voltage or drain-source voltage.
As a method for preventing this, in a high side switch circuit employing an emitter follower or source follower circuit of N type elements such as NPN transistors or N channel FETs, a circuit construction may be adopted wherein a drive power source is provided for base drive or gate drive independently of the main circuit, or, alternatively, in which the drive power source is stepped up in voltage from the main power source voltage by an amount matching the base-emitter voltage or gate-source voltage.
However, a large number of circuit elements are required in order to construct such an insulated power source and/or voltage step-up circuit, so this itself increases the cost of the device, and, furthermore, the failure rate will be increased in proportion to the increase in elements: thus the reliability of the circuit is lowered.
In contrast, FIG. 12 shows a circuit layout, called a low side switch, in which switching of the load current is performed on the earthed side of the load.
In this circuit, a construction is adopted wherein transistor TR1 constituting the switch means is provided on the earthed side of the load and the PWM signal of predetermined duty ratio is applied to the base of this transistor TR1; the circuit can therefore be made of even simpler construction than the high side switch.
When the PWM signal becomes high level and transistor TR1 is turned ON, load current from the power source flows to earth through load L and transistor TR1. When this happens, due to the characteristics of the load L, the load current increases with time, finally becoming constant at a saturation point given by the source voltage and the amount of resistance, not shown, in inductive load L. The chopper control method, whereby a mean current value lower than this saturation point that is required for load L is maintained is essentially the same as described above with reference to the high side switch.
This method is superior in that it can be implemented with a simpler layout than the high side switch, and in that N type semiconductor elements can be employed for the switching elements, the drive voltages of these N type semiconductor elements being always referred to earth, so the emitter or source potential is fixed, etc., but it suffers from the problem that the current cannot be cut off if part of the load gets short-circuited to the earth side.
Apart from this, there is the prior art example of Early Japanese Patent Publication No. H5-57918. This example is constructed as shown in FIG. 13: power source E2 is connected to the gate of N channel FET Sw1 through resistor R1 and diode D1 and a transistor switching element TR1 is provided. In addition, a capacitor C1 is provided at the point of connection of resistor R1 and diode D1. When transistor switching element TR1 is ON, this is charged up, but when it is OFF it is discharged, supplying charge to the gate. The N channel FET can thereby be controlled at the power source side of the load.
However, as can be seen from FIG. 13, this is subject to the problems that a separate power source E2 is required to supply charge to the gate, and, when transistor switching element TR1 is turned ON, apart from the current from this power source E2 that charges capacitor C1, current flows to earth through resistor R1 and transistor switching element TR1, so the efficiency is not necessarily good. If the resistance value is made large in order to reduce the current flowing through this resistor R1, there are problems such as that the charging resistance between gate and source when N channel FET Sw1 is ON becomes large, lowering the speed of switching of the FET.
Apart from current control by digital switching as described above, the method is available of direct control of the current amount in analogue fashion.
FIG. 14 is a block diagram of a conventional analogue control type constant current circuit.
This circuit is constituted of: a current detection circuit 1; a drive control circuit that performs current control by a method such as changing the base voltage of transistor Tr1 in response to the detected current value of current detection circuit 1; and a bipolar transistor Tr1 and power source 3. In comparison with chopper control, this method is not subject to problems such as radiation of unwanted electromagnetic waves from the output wiring or generation of inductive noise. Also, in principle, constant-current control can be performed without special means even if there is a short circuit in the load wiring during driving of the load. However, in practice, the load current flows in a condition with the current control element having applied to it the potential difference between the power source voltage and output terminal voltage, so the loss of this element is considerable, making it necessary to provide some way to emit the amount of heat that is generated by this element under these conditions: this leads to problems in regard to cost and reliability.
As described above, with the conventional inductive load drive devices, in the case of current control using digital switching, if a high side switch (which is safe in regard to short circuiting during operation of the load) is constructed using P type elements, although the circuit construction is simple, the element unit cost is high and the element characteristics poor; whereas, if the switch is constituted using N type elements (which have superior element characteristics), the number of such elements is increased, a separate power source is necessary, and the switching rate is lowered: thus, in either case, there were problems in regard to cost and reliability. And if analogue current control was used, the element loss during operation was high, so that elements were required that could tolerate large loss and, furthermore, some way for the device to emit the heat was required.
A first object of the present invention is therefore to constitute a high side switch of good efficiency using N type elements by means of comparatively few circuit components. A second object is to reduce as far as possible the amount of heat emitted by the elements of an analogue control type constant current circuit by controlling the voltage applied to these elements whilst maintaining constant current control.